The fringing reeds (Phragmites australis (Cav.) Trin. ex Steud., Poaceae) at Lake Constance-Untersee were mown in winter thereby drastically reducing the input of decomposable organic matter (OM) to the sediment. A 4-years monitoring program was started to test whether or not this management influences the hydrochemistry of the bottom water layer, and leads to a decrease in OM, total N and total P in the surface sediment layer. It was found that the sediment/ water system of reed beds is well buffered against the removal of the current year's crop of dead Phragmites straw, because (I) the yearly input of dead biomass contributes only 14% to the OM pool of the reedbed, and (II) the decomposition rate of Phragmites litter is very low (c. 0.9 mg g-1 d.m. d-1).